(1) Field of Invention
The invention relates to a process for applying a coating comprising a polymerizable composition to a metal strip that will be further coiled and destined to be post-formed.
Coil coaters coat metal coils of various types and forms. These metals are selected for various end uses depending on their specific characteristics. There is a broad diversity of applications such as container products including beer and beverage cans, building materials such as ducts, roofing, rain gutters, furniture, transportation applications like automotive trim and travel trailers. In the case of some metals such as steel, a zinc protective layer against corrosion can be applied previous to the coil coating process. It is usually done on a galvanization or electrozinc line. The bare metal coil is unwound and both top and bottom sides are cleaned to remove dirt, oil, grease, surface rust and prepare a chemically clean surface. The zinc layer can be deposited by dipping the strip in a zinc bath (galvanization process) or by electrodeposition (electrozinc process). The strip is then oiled before being recoiled in order to protect the metal surface against corrosion during transportation and storage before coating. Coil coating most often takes place on another line. It is a continuous and highly automated process for coating metal at high speed before fabrication. The process usually consists of the following steps. The metal coil is unwound again and both the top and bottom sides are mechanically and chemically cleaned to remove oil, grease and dirt. After cleaning, a chemical pretreatment can be applied to the sheet in order to enhance the corrosion protection and the paint adhesion. The strip then moves directly into a drying oven before entering the coating unit. After the paint has been applied, the strip moves again into a drying oven before being cooled and rewound for shipment.
The precoated sheets are used in the metal-processing industry. So, many metal articles are not coated or painted as used or when assembled, but are first coated as flat stock provided in coils and then formed, cut and assembled into the desired articles. As a consequence, a key criterion in almost all cases is the ability to post form or bend the coated strip. Post forming is usually done by high speed processing equipment wherein the metal bending can be very severe. The coating must be flexible to allow the post forming or bending to occur without cracking and yet maintain adhesion. The full system what means the metal, in some cases the protective zinc layer, the pretreatment layer and the paint layer has also to reach a high level of corrosion resistance. Basic properties like chemical and stain resistance remain important and for some applications, properties like humidity and sterilization tests resistance are also required. Radiation curable systems could be a good fit for coil coating; the substrate is flat, the line speed is high, it can solve environmental issues like the use of solvents and it requires less energy and floor space than the curing ovens necessary with other coating technologies. However until now, radiation curable coating compositions, especially UV formulations have not been widely used for coil coating applications. Typical UV formulations consist of (meth)acrylated oligomers and reactive diluents, which when cured, form a highly crosslinked coating layer having good properties such as chemical resistance, scratch resistance and surface hardness. Such high crosslink density is also associated with a limited flexibility and shrinkage of the film limiting adhesion on the metal substrate. When trying to find solutions to these stringent requirements, one is often limited between lowering the molecular weight to achieve a reasonable viscosity and the need for a substantive molecular weight between crosslinks in order to reach the required cured coating flexibility.
(2) Description of Related Art
For coil coating applications where a high level of flexibility is essential for post forming operations, such typical UV formulations are not suitable. US 2002/0132059 A1 describes the principle of using a photopolymerizable coating composition for the coating of a coiled metal sheet. In the method for applying a coating to coiled metal sheet therein described, the metal sheet is first cleaned and then pretreated before applying the coating composition. No specific photopolymerizable compositions are disclosed in this patent application and almost any composition comprising a difunctional unsaturated oligomer, a monofunctional unsaturated acrylic monomer, a difunctional unsaturated monomer and a photoinitiator seems to satisfy the requirements. Example 1 of this patent application refers to a difunctional unsaturated oligomer having a polybutadiene backbone. U.S. Pat. No. 3,652,520 discloses such oligomers and more specifically acrylated polybutadiene resins obtained from the reaction of hydroxyl terminated polybutadiene and (meth)acrylic acid or derivatives thereof such as acryloyl chloride, methylmethacrylate and methylcrotonate. These oligomers do not permit to obtain good results in coil coating applications. Hence not all photopolymerizable coating compositions comprising difunctional unsaturated oligomer, a monofunctional unsaturated acrylic monomer, a difunctional unsaturated monomer and a photoinitiator permit to obtain satisfying results when used in coil coating processes. US 2003/00018150 describes a radiation curable composition comprising a blend of a polybutadiene oligomer, an acrylated bisphenol-A derivative and a reactive compatible compound and its use for coil coating application. These blends are obtained by mixing the different ingredients. These blends are not storage stable and demix quite rapidly.
Hence, the known compositions do not permit to obtain satisfactory results when used for coil coating applications.